Q: What are the basic guidelines one should keep in mind while
designing a database? Is denormalization always good?

In simple words.
The reason to normalize is don't duplicate data fooly
The reason to denormalize is don't get you boss kick your ass, because a
query is taking too much time.

There is a limit when tuning, when you can't get more performance, unless
you denormalize, buy hardware or move to RAC.

The guideline is performance, and common sense.
If denormalizating it takes 0,03 s and without denormalizating takes 2s
without taking in count that it increase 10x the amount of block read, etc.
..and this is a process t hat run 1000's of time a day, don't denormalizing
is a mistake.
..but if this process is run few times a day, denormalizing is a mistake.

4th and 5th normalization is not always advisable, all depends.

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